One known example of temperature-detection circuits employing thermistors is shown as an equivalent circuit in FIG. 19, where an input terminal electrode 24, a resistor 26, an output terminal electrode 23, a thermistor 22, and a ground terminal electrode 25 are connected in series in that order.
In the temperature-detection circuit with this structure, voltage is applied between the input terminal electrode 24 and the ground terminal electrode 25, and then the voltage between the output terminal electrode 23 and the ground terminal electrode 25 is measured. Thereafter, the output voltage is converted into a temperature to detect a change in the temperature.
In order to reduce the size of the temperature-detection circuit with the above-described structure, it is a typical approach to mount components including the resistor and thermistor on a circuit board.
However, if the size of the temperature-detection circuit is reduced by the above-described approach, manufacturing takes more time, leading to high manufacturing cost. Furthermore, since the above-described approach leads to a large mounting footprint on the circuit board, the size cannot be reduced more than expected.
Furthermore, in order to achieve small size and integration into a single chip, there is proposed a composite device including a chip thermistor element, terminal electrodes formed on both end surfaces of the thermistor element, and a resistor layer formed on a side surface of the thermistor element, wherein one terminal electrode, the resistor layer, the thermistor element, and the other terminal electrode are connected in series in that order (for example, Japanese Unexamined Patent Application, Publication No. H10-294207).
In the composite device with this structure, components including the thermistor element, the terminal electrodes, and the resistor layer can be integrated into a single chip to reduce the total size. Furthermore, the mounting footprint on the circuit board can be made small, thus allowing the total size of the circuit board to be reduced.
However, for the composite device with this structure, a resistor is separately required to achieve a linear characteristic of the thermistor element. Furthermore, it is necessary to select the resistor or add a trimmer in order to achieve matching between the thermistor element and an internal resistor. This leads to high manufacturing cost.